The Study Of Embedded DDS And Its Application In Video Encoder

With the development of modern electronic systems, frequency synthesizer is required to have higher performance. Frequency synthesizer is the most important part by which performance of all electronic systems is determined. The objects of frequency synthesis technology include low phase noise, pure output spectrum, fast frequency switching speed, high frequency resolution, etc. Direct digital synthesis (DDS) is a new frequency synthesis technology coming after direct frequency synthesis (DS) and phase locked loop frequency synthesis (PLL).Compared with other traditional frequency synthesis technology, DDS has many own merits. It is widely used in such fields as radar, communication, electronic warfare and electronic measurement instrument.This subject derives from the project of digital video encoder in CETC the 58th research institute. The digital signal mixer in video encoder is composed of frequency synthesizer, adder and multiplier. Color difference (UV) components are modulated on to the sine and cosine components; these quadrature components are added to create a Chroma component. DDS must be needed to fulfill the requirements of digital video encoder with different broadcast standards for carrier frequency, resolution and SNR because it has many merits.Basic principle and logic structure of DDS are introduced in this paper. The relations of several important parameters are expressed in some formulas. Many merits of DDS as a new frequency synthesis technology are summarized. New structures of coordinate DDS and DDS+PLL system which can avoid defects of low output frequency and bad spurious performance are studied .Then ideal and actual DDS output spectrum are analyzed and simulated. Sources of spurious signals for entirely digital structure of DDS include phase truncation error, amplitude quantization error and non-linear characteristics of DAC. Phase dither technique as one of the spur-reduced method is effective on improving DDS output spectrum purity.Some ROM compression methods and CORDIC algorithm for DDS of look-up table are analyzed. They can reduce the defects of large size ROM such as delay, high cost and low reliability. Every parameter of look-up table DDS module is determined by the requirements of video encoder to carrier wave. Some novel structures is exploited in place of traditional DDS module. These methods include sine function symmetry, spur-reduced technique and sine function approximation algorithm. The results of simulation are given to testify their usability. Their own merits and defects are summarized.